Apologies for that.

I purchased Corsair Vengeance LPX. If I must return this for crucial, i'll do that.

CMK32GX4M2A666C16

Also, thank you so much for the prompt response. I also did attempt to take the battery out but maybe I did not leave it out long enough.

Actually you can clear the CMOS by pulling the CMOS battery and leave it out for 15 minutes. The procedure is here in the service manual. Now as for the RAM, the absolute most compatible RAM is from Crucial. You did not specify the brand you purchased but it is possible that the RAM is not totally compatible.

I returned the corsair and purchased Crucial as per recommendation and am still having the exact same problem.

Crucial Ballistix 3200 DDR4 16gb x2. CL16 BLK16G32C16U4B

Only one stick will work at a time, both work when tried individually but ONLY with the original 8gb (OEM) stick in slot 1 and a 16gb(Crucial) stick in slot 2. If a 16gb stick is in slot one it will not work. It will also work if the 4gb (OEM) stick is in slot one with a 16gb in slot 2. There is no way to have both slot 1 and 2 with 16gb sticks.

I'm also seeing the exact same issue with this model PC and using Crucial memory (CT2K16G4DFD8266).  The 8Gb stick that came with the PC must be in one of the 2 slots in order for the PC to boot.  Yet a 2x 16GB configuration is listed as a supported configuration.
When power is applied, the power led starts to blink 2 times (amber) followed by 5 times (white).  This is listed in the manual as "invalid memory installed".  Yet the PC will boot and recognize either of the 16GB sticks as long as that 8GB stick is installed.

There are multiple posts regarding this issue and they all seemed to imply that it was due to a BIOS upgrade.  My computer came with 1.3.1 on it and I just updated it to 1.5 and still had the exact same issue.

Come on Dell, what gives?

I am having this same issue as well. I've read that I should downgrade my bios, but to what version?

https://www.dell.com/community/Inspiron-Desktops/Memory-cant-use-after-bios-update/m-p/8104718/highlight/true#M31542

My Problem is exactly this also and the only help ive gotten thus far is to contact sales and purchase ram directly through them.

Anyone figured out a solution to this?

I keep getting a message from Dell support saying that they can't help me with other companies' products and direct me to their sales team to purchase 1 x 16GB stick for $160...

They claim to be looking into the issue, but that has been the response that I've been given for the last ~2 weeks now.  If/when I get any kind of actual update, I'll be sure to post it.